Keith Mack
In chemistry, the rate law refers to the rate of a reaction in terms of the concentrations of the reactants. Pure solids and liquids are generally excluded from rate laws because their concentrations are assumed to be constant. This is because, as a solid or liquid reactant is consumed, the total amount decreases but its concentration generally does not. However, the reaction rate can depend on the surface area of the solid exposed to reactants, and how this surface area changes over time.
| Characteristics | Values |
|---|---|
| Pure solids and liquids are excluded from rate law expressions | Their concentrations do not change |
| The rate of reaction | Depends on the surface area of the solid exposed to reactants |
| The forward rate of the reaction | Rate = constant x constant, which is just rate = constant |
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What You'll Learn
Pure solids and liquids are excluded from rate laws
However, this law only applies to substances that are in the gas phase or are dissolved in solution. Pure solids and liquids are not included in the equilibrium expression because their concentrations do not change during the reaction. The concentration of a pure solid or liquid is essentially its density, which remains constant regardless of the amount present. This is because the particles in a solid or liquid are closely packed together and do not have the freedom to move and spread out like particles in a gas or solution.
The activity of pure solids and liquids is considered to be one. In thermodynamics, the activity of a substance is a measure of its 'effective concentration' in a mixture. For pure solids and liquids, this is always one, regardless of the actual amount present. This is another reason why they are not included in the equilibrium expression.
The reaction rate for solids also depends on the surface area of the solid exposed to reactants. How the surface area changes over time depend on the geometry of the material.
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The forward rate of a reaction involving solids and liquids
The rate of a reaction refers to the speed at which products are formed from reactants. It provides an insight into the time frame under which a reaction can be completed. The rate of a reaction involving solids and liquids is dependent on various factors.
Pure solids and liquids are generally excluded from rate law expressions as their concentrations remain constant. This is because, although the total amount of a pure solid or liquid decreases as it is consumed in a reaction, its concentration does not change. Therefore, the forward rate of reaction involving pure solids and liquids can be expressed as a constant.
However, the reaction rate of solids and liquids can be influenced by certain factors. For instance, the surface area of the solid exposed to reactants affects the rate of reaction. A larger surface area of a solid reactant will lead to an increased reaction rate. This is because a larger surface area provides more opportunities for the reactants to come into contact and collide, facilitating the reaction. Similarly, the geometry of the solid can impact the rate of reaction, as different shapes of solids will result in varying surface area exposures over time.
Additionally, the presence of a catalyst can significantly influence the forward rate of a reaction involving solids and liquids. A catalyst provides an alternative reaction pathway with lower activation energy, increasing the speed of both the forward and reverse reactions. Furthermore, changes in the concentrations of reactants and products, as well as alterations in temperature and pressure, can also affect the forward rate of a reaction.
It is important to note that the rate law for a reaction can be determined by studying the initial instantaneous rate of reaction under different initial concentrations of reactants. This allows for a better understanding of how the reaction rate may change over time.
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The kinetics of solids and liquids
However, it is important to note that the reaction rate of solids can depend on the surface area exposed to reactants. The geometry of the material will determine how the surface area changes over time. For example, the kinetic energy and speed of liquid molecules depend on their temperature. The molecules in a liquid are in constant motion, and as the temperature increases, so does the kinetic energy. This is similar to gases, but unlike gases, there is very little empty space between the molecules in a liquid. Liquids are much denser than gases and are virtually incompressible.
The molecules in a liquid are held together by strong intermolecular forces, which prevent significant expansion when heated. This is in contrast to gases, where the distance between molecules is very large compared to the size of the molecules, resulting in low density and high compressibility. The kinetic molecular description of liquids must consider both the nonzero volumes of particles and the presence of these strong intermolecular forces. Due to these forces, the molecules in a liquid are packed tightly together, resulting in short-range order.
While the molecules in a liquid move rapidly with respect to one another due to their higher kinetic energy compared to solids, their arrangement is not completely random. This is because the strong intermolecular forces in liquids prevent the molecules from assuming a completely disordered state.
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The surface area of solids and its effect on reaction rate
The rate of a chemical reaction is influenced by the surface area of the solid reactants involved. This is because solid and liquid reactants have constant concentrations, and the reaction occurs at the interface of the two phases, i.e., on the surface of the solid. As a result, the larger the surface area of the solid, the faster the reaction will be.
When a solid reactant is exposed to a liquid reactant, the reaction can only occur where the two substances come into contact—at the surface of the solid. By increasing the surface area of the solid, more molecules are exposed, providing more locations for the reaction to take place. This increases the likelihood of particle collisions, leading to a higher reaction rate.
For example, consider a loaf of bread. If you slice the loaf in half, you effectively double the surface area, providing more areas for potential reactions. Similarly, by cutting a solid reactant into smaller pieces or grinding it into a powder, the surface area increases, and the reaction rate rises. This is because the smaller particles have a greater total surface area, exposing more molecules to the reaction.
The effect of surface area on reaction rate is not limited to solids. Powders, due to their higher surface area compared to blocks of the same mass, also react faster. This is because the reaction rate depends on the exposed surface area, and powders offer more surface area for potential reactions.
In summary, the surface area of solid reactants plays a crucial role in determining the rate of a chemical reaction. Increasing the surface area, whether by cutting or grinding the solid, exposes more molecules, leading to a higher chance of particle collisions and, consequently, a faster reaction.
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The role of solids and liquids in dynamic equilibrium
Pure solids and liquids are typically excluded from rate laws because their concentrations remain constant. This is due to the fact that the concentration of a pure solid or liquid is defined as 1, and does not change with the amount present. In other words, increasing the amount of a pure solid or liquid does not change its concentration, as it is not dissolved in a solvent.
However, solids and liquids do play a crucial role in dynamic equilibrium. Dynamic equilibrium refers to the state where the measurable properties of a system, such as pressure, density, colour, or concentration, do not undergo any further noticeable changes over time. In the context of solids and liquids, dynamic equilibrium can be observed in various physical processes, such as phase transformations.
For example, ice and water can exist in dynamic equilibrium at a specific pressure and temperature, known as the normal melting or freezing point. At this point, both the solid and liquid phases occur simultaneously, with the rates of the forward and reverse reactions being equal, resulting in a constant amount of ice and water. This is an example of solid-liquid equilibrium.
Additionally, dynamic equilibrium can also be observed in systems involving the dissolution of solids or gases in liquids. For instance, when dissolving sugar in water, a dynamic equilibrium is established between the solute molecules in the solid state and in the solution, with the rate of dissolution being equal to the rate of crystallisation. Similarly, when opening a carbonated drink, a new dynamic equilibrium is established as the drink adjusts to the lower pressure of the atmosphere, resulting in the escape of some CO2 gas.
In summary, while pure solids and liquids are typically excluded from rate laws due to their constant concentrations, they play a crucial role in dynamic equilibrium. Dynamic equilibrium involving solids and liquids can be observed in phase transformations, such as the melting and freezing of ice, as well as in the dissolution of solids or gases in liquids, such as the behaviour of sugar or carbon dioxide in water.
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Frequently asked questions
No, rate law does not apply to liquids and solids because their concentrations are assumed to be constant and unchanging.
Liquids and solids are excluded from rate law because their concentrations do not change.
The reaction rate of solids depends on the surface area exposed to reactants.
The change in surface area depends on the geometry of the material.
No, in textbooks like Chemistry by Zumdahl, Raymond Chang, and Principles of Molecular Chemistry, there are no examples of rate law expressions with solids and liquids.
